1. Field of the Invention
This invention relates to the removal of hydrogen sulfide from gases, and more particularly to a process for removing hydrogen sulfide from hydrogenated Claus process tail gas in which the hydrogen sulfide is converted to elemental sulfur.
2. Description of the Prior Art
With the increasing concern over the pollution of the atmosphere, great demands have been made on industry to produce pollution-free products and to do so in a pollution-free manner. One area of particular concern has been the release of sulfur and its compounds into the atmosphere during the refining of petroleum, the sweetening of sour natural gas, the processing of ore, the destructive distillation of coal, and during other processes which produce sulfur-containing gases.
The Claus process, which is widely employed for the desulfurization of hydrogen sulfide-containing gases, is about 90 to 97 percent effective in converting hydrogen sulfide to elemental sulfur. While substantial amounts of sulfur are recovered and, thus, prevented from being released into the atmosphere, in many instances it is necessary to further treat the Claus process tail gas to make it acceptable for discharge into the atmosphere. One process particularly useful for treating these tail gases is the Beavon sulfur removal process described in U.S. Pat. No. 3,752,877, which process involves the hydrogenation of the Claus process tail gas to convert the various sulfur compounds to hydrogen sulfide, and then employing a process, known as the Stretford process, to remove the remaining hydrogen sulfide. The Stretford process is a well-known process for converting hydrogen sulfide to elemental sulfur and is described in U.S. Pat. Nos. 2,997,439, 3,035,889, and 3,097,926.
However, while the Stretford process is satisfactory in many applications, it has been discovered that one of the principal constituents of the washing solution, anthraquinone disulfonate, can be chemically consumed, particularly when the process is used to treat hydrogenated Claus process tail gas. When the anthraquinone disulfonate is chemically consumed, frequent addition of this expensive chemical is necessary to maintain the proper concentration of this constituent in the washing solution.
It was known that during the operation of some Stretford units, such as those used to treat coke-oven gases, sodium thiocyanate is naturally produced in the washing solution as a by-product; while in other Stretford units, such as those which treat hydrogenated Claus process tail gas, sodium thiocyanate is not produced as a by-product of the process. In those Stretford units wherein sodium thiocyanate is naturally produced, the thiocyanate ion concentration of a fresh washing solution is initially zero, and over a period of many months of operation slowly increases to a concentration of 150 grams per liter or more.
It has now been discovered that the presence of thiocyanate ion in the washing solution inhibits the chemical consumption of anthraquinone disulfonate (ADA), and that by increasing the concentration of thiocyanate ion in the washing solution, the rate at which the ADA is consumed can be reduced.
Accordingly, a principal object of this invention is to provide an improvement in the Stretford process for the removal of hydrogen sulfide from gases and its conversion to elemental sulfur.
Another object of this invention is to provide an improvement in the Stretford process when used to treat hydrogenated Claus process tail gas.
Yet another object of this invention is to reduce the amount of ADA consumed in the Stretford process when used to treat a hydrogen sulfide-containing gas.
A still further object of this invention is to reduce the amount of ADA consumed in the Stretford process when used to treat hydrogenated Claus process tail gas.
Further objects and advantages of this invention will be apparent to those skilled in the art from the following description.